Balance selection: frequency dependent, heterozygote advantage

Frequency-Dependent Selection and Heterozygote Advantage are two type of balance selection that contribute to maintaining certain phenotypes in an oscillating equilibrium.

Frequency-Dependent Selection

In regions of tropical rainforests where toxic butterflies exist, there is often a presence of non-toxic butterflies that mimic the patterns of the toxic ones. These non-toxic butterflies deceive birds into perceiving them as toxic. The proportions of these butterflies constantly fluctuate within a certain range. When the population of non-toxic butterflies becomes too high, it misleads birds into perceiving a decrease in toxicity and increase predation. However, the low population of toxic butterflies reduces the probability of being captured, giving them a reproductive advantage. As the toxic butterflies reproduce in excess, the birds resort to eating other insects. This results in a decrease of selection pressure for non-toxic butterflies and their population becomes big again, initiating a new cycle of natural selection.

Heterozygote Advantage

Sickle cell disease occurs in regions plagued by malaria. The gene A for hemoglobin is mutated to produce a recessive allele, gene a. Heterozygotes (Aa) can resist malaria without significant health issues, while homozygotes (AA and aa) are both unable to resist malaria. Homozygotes with aa also suffer from severe anemia, blood clots, and other diseases. Clearly, the Aa genotype is more advantageous for survival. The frequency of the a allele increases, but it does not completely eliminate the A allele like in directional selection. The recessive allele requires A gene to show its advantages. Furthermore, not all individuals with the AA genotype die from malaria. In fact, they have some resistance to the disease, and the presence of the a allele enhances this resistance. Eventually, the a allele will stabilize at a certain frequency.

Frequently Asked Questions

Malaria-resistant heterozygote vs homozygotes

There is a few variant proteins in the red blood cells of heterozygotes to make the red blood cells rupture prematurely, and release immature malaria parasites into the blood. They are easily engulfed and destroyed by phagocytic cells. In malaria-endemic areas, heterozygous individuals have a higher chance of survival.

However, heterozygotes have a disadvantage in regions with good sanitation conditions. Some of their offspring are aa genotype and are less likely to survive. Due to the presence of a few variant proteins, heterozygotes have a lower oxygen-carrying capacity compared to purebred AA individuals. Their relatively poorer physical condition makes them more likely to fail in competitive situations.